Electronic fast-time constant switch



Sept 1 7,

1957 D. w. ADAMS ELECTRONIC FAST-TIME CONSTANT SWITCH Filed July 16.1954 -11 Z5 L3 36 5 7 /6 2 A /4 Mu/ff- Mu/zfwar-afar w'brafor' INVENTOR.00/6 IZ Ada/77$ United States Patent OfiFice 2,806,948 Patented Sept 17,1957 This invention relates to radar signal receiving circuits, and moreparticularly to an improved electronic fast-time constant switch circuitwhich may be employed with presently existing radar receiving sets, andwhich may be incorporated as a modification in radar receiving sets ofcurrent design, or which may be built internally into new radarreceiving sets.

A main object of the invention is to provide a novel and improvedelectronic fast-time constant switch for use with a radar signalreceiving circuit of the type having a relatively long time constant inorder to provide clearer detection of targets and to more clearlydistinguish between target indications and spurious indications, such asare obtained from clouds, ground returns, or other obstacles.

. A further object of the invention is to provide an improved electronicfast-time constant switch for use with radar signal receiving circuits,said switch involving relatively simple components, being reliable inoperation, and operating to clarify an indication produced by a targetby positively discriminating between the indication produced by thetarget and the spurious indications which may be produced by groundreturns, clouds, and the like.

A still further. object of the invention is to provide an improvedelectronic fast-time constant switch device for use with radar signalreceiving circuits of the type having a relatively long time constant,said switch device being adapted to be connected across the outputterminals of the receiving circuit to separate radar signals fromspurious signals, such as ground returns, and the like, the improvedswitch device involving inexpensive components, being dependable inoperation, and enabling the indication produced by a target, even itrelatively small, to be distinguished from spurious indications, such asthose produced by ground returns.

Further objects and advantages of the invention will become apparentfrom the following description and claims, and from the accompanyingdrawings, wherein:

- Figure 1 is a wave-shape diagram illustrating the voltage wave-timepattern of normal video signal information passing through aconventional long time constant resistancecapacity circuit, such as isemployed in a radar signal receiving system.

Figure 2 is a wave-shape diagram similar to Figure '1 and showing thevoltage wave-time pattern of the signal of Figure 1 when passing througha radar circuit provided with an improved electronic fast-time constantswitch according to the present invention and illustrating the man nerin which the ground return signal is separated from the indicationproduced by a target.

Figure 3 is a schematic wiring diagram, partly in block form,illustrating an improved electronic fast-time constant switch accordingto the present invention and showing the manner in which the electronicfast-time constant switch is connected to the output circuit of a radarsignal receiving set.

Figure 4 is a schematic wiring diagram of one of the multivibrato'rsemployed in the electronic fast-time constant switch arrangement ofFigure 3. i

all

A prime purpose of the present invention is to provide a relativelysimple and'compact unit, employing few parts, which is especiallysuitable for attachment to already existing radar sets, or which may beincorporated into the designs of new radar sets, the device beingintended to provide a clearer detection of targets by separatingspurious indications, such as those obtained by ground returns, clouds,and similar obstacles, from the indications obtained from the targets.Referring to Figure 1, a typical voltage Wave-time pattern isillustrated, this pattern representing the voltage wave passing throughany conveniently sei lected, long time constant, resistance-capacitycircuit in a radar receiving set, for example, the circuit shown inFigure 3 which includes the capacitance 6 and the resistor 7, theresistor 7 being connected across the output terminals of the selectedportion of the radar receiving set, one of the output terminals beingshown at 8, and the other output terminal constituting the ground, shownat 30. The ungrounded input terminal of this portion of the radarreceiving set is designated at 5.

As shown in Figure 3, the electronic fast-time constant switch,designated generally at 31, is connected across the output terminals ofthe above described portion of the radar receiving set, for. example,between a point 9 along the output conductor 32 and ground, designatedby the reference numeral 34.

In Figure l, the reference numeral 1 designates the main orsynchronizing triggering pulse which starts the sweeps of the radaroscilloscopes. Designated at 2 is the video signal produced by a cloud,ground return, or similar obstacle. Designated at 3 is the video signalproduced by a target, such as an aircraft, or the like, which is flyingabove the ground which gives the spurious ground return signals.

As will be apparent from Figure 1, whenever the video indicationproduced by the target, namely, the wave element 3, is relatively small,as compared with the video signal produced by the ground return, namely,the signal which produces the wave element 2, it is very difficult todistinguish between the target indication 3 and the ground returnindication 2. A main purpose of the present invention is to overcomethis difiiculty.

Referring now to Figure 3, the electronic fast-time constant switch 31comprises an input terminal 12 which is connected through a conductor 35to the input of a first multivibrator 13. The output conductor 36 of themultivibrator 13 is connected through a condenser 14 to the inputconductor 37 which is connected to a second multivibrator 16. A resistor15 is connected between the conductor 37 and ground, thereby defining a.ditferentiating circuit, in conjunction with the condenser 14, betweenthe output of the first multivibrator 13 and the input of the secondmultivibrator 16. The output wire 38 of the sec- 0nd multivibrator 16 isconnected through a relatively large condenser 18 and a resistor 19having a relatively high resistance value to a conductor 39. Conductor39 is connected to the ground point 34 through a relatively lowresistance 17. Connected between the point 9 on the radar output circuitconductor 32 and the conductor 39 is .a first rectifier 10 which ispolarized to allow electrons to flow from the conductor 39 to theconductor 32, namely which is polarized so that current can flowtherethrough from conductor 32 to conductor 39 only when the junctionpoint 40 between resistor 17 and conductor 39 is at a lower potentialthan the connection point 9. Designated at 41 is the common junctionpoint between the condenser 18 and the resistor 19. Connected betweenthe junction point 41 and ground is a second rectifier 11 which ispolarized to allow flow of electrons only in the direction of the arrowshown adjacent the rectifier 11, namely, from the junction point 41 toground. In ether words, current will flow from ground/to, junction point41 when the potential of the junction point 41 drops below groundpotential.

Thezrectifiers rlitl a-nd i1] 2 can: be dio'desptriodegiseleniumrectifiersg or the: like; anchareiillustrated 1withiconneotions':

' which assumesythe video. appliedizat the input terminal.

5:t'o beiofppositiver polarity: The zarrowszloeate'dmdjacenta therectifiers in'zFiguredtshowthm directiomofithe-eelec-r tron flow Lwhemthe; rectifiers af6 Cndi1Ciil1g; Thehconnections of;v the rectifiersiwill bearevers'edaif the video ap r plied to; the input iterminal -fisrof: a;=nega.tive. polarity.

. QOndenSeTr- 1'82 and' resistor .19*.'ar.'e..of largewaluesr'toprefvent the=condenservfrom=fullychargingnz Thus'raiposi-s .tivevol-ta'geaisnorrnally:droppedziacrosse-ther:resistors 1-71 and. 19:;Resistorr17 hasiaasmall.resistanceaso that itcan reduce thetime;constant=.of:theqresistance-oapacity circuit structionand includesa-firsttri ode 52-interconnected in the manner shown with-asecondtriode. 54: The multivibrator it has ;the; input terminal: 55 andthe output terminal 56:

The 'triode' 52 is normallycut 011- by fixed negative bias-- I providedby the battery 51.v A positive triggering'pulse may be-appl ied to theinpnt termin'al 55; which:will produce.-a-square:wave at the outputterminal 56 whose length canbe varied by the operator bymeans ofa-variable grid resistance 57' employed with the triode 54, so that the,

duration of the square wave 1 generated by the" first multivibratorcan-be regulated in accordance with the operators-choice u 1 The outputof the first rnultivibrator 13, consisting ofa square wave, is sentthrough the differentiating; circuit defined-by-thecondenser 14 and theresistor 15. The trailingredge of this square'wave acts as a shorttrigger whichoccurs at a time value corresponding to the-point 4 shownin Figure 2 and which acts 'to trigger the secondmultivibrator 16;

The output of the second-multivibrator 16 is a short pulse of the orderof one ortwomicroseconds'in duration,- thispulse being of .a' negativepolarity (when the video appliedto'the input terminalS is of positivepolarity); The. output pulse ofthe'secondmultivibrator 16 is' positivein polarity whenthe video applied to the input terminal 5 is negative.Inthe arrangement illustratedinFigure 3 thevideosignal is assumed to bepositive, and-hence,r the pulseprovided bythe second multivibrator 16'is negative in'polarity. It willbe understood-that the positive (ornegative) 7 incomingtriggering pulse 1 isapplied to the input terminaIJZ in Figured, this pulse acting to trigger the first multivibrator 13.

Resistor 17 'normally has a positive potential-existing thereacross,namely the junction point 40 is positive wit'n respect: to ground;because condenser 18 is charging throughthe resistor 17, as well-asthrough the resistor 19, so' that no electrons-flow through therectifier 10.

When the .secon dmultivibrator l dischargesits negativelsquare pulse,the condenser 18 quickly discharges through the, rectifier 1 1, reducingthe potential across resistor 17to zero, namely, bringing thejunction-point 40 to ground potential, allowing electrons to flowthrough the rectifier'slfi and through the resistor 17; for a shortperiod of 'th'eordr' of lone ortwo .microseconds,'correspending tothelength of thetnegativepulsedelivered by themultivibrator- 15.Since'electrons can-flowlthrough the rectifier lfljthe time constant oftheresistanc'e-ca pacity circuit' defined by condenser-6 and resistor 7is reduced; said 'freduction Y occurring at" a time corresponding to thepoint 4 on: the voltage .wave diagram of Figure 2.

Whenever the time constantuof the resistance-capacity. circuit definedby condenser6 and resistor 7 is reduced, as at the point 4 in Figure 2-,the video of theground returns, shown at 2 in Figures 1 and 2, isdifierentiated, but not the video 3 of the target. This separates theindica tions of the target and the ground returns and gives a 7 fullindication of the target.

It will beunderstoodthat condenser, '18 and resistor 19 2 are of largevalues to prevent the condenser 18 from fully charging. Thus; under"normal conditions asubstantial positive voltage exists acrossthe seriescircuit defined by the resistors 17 and 191 Thus, the side of condenser18 connected to the junction'-point=41-- is at a high positivepotential. When the negative pulse is generated at the output of themultivibrator 16,'the opposite side of the condenser is, droppedsuddenly in potential by the application "thereto 'ofithenegativejpulse; causing fthec'o'ndenser to dischargerapidly].Thisidiscliarge'.takes place" the potential across-the resistor17 to zero; Thus; for the shortp'e'riod ofthe 'orde'r of onefor two-rnicrosec'onds, namely; for the length of the negative pulse, theresistance capacity circuit defined by condenser 6-'and res istor 7operatesasif it had a" short; time" constant; whereby the. groundreturnindications are' ditferentiated :but not" the videoindicationsfiprodiicedbythetarget.

' Byiadjusting the variable resistor- 57 of the jfirst multivibrator13j'the time point at whichth'e differentiation 4 maybe madel to occurat any location" onithe "sweep V chosen by the operator;

While a specific enib odirrrent ofarrirnproved ilectronic I i fast-timeconstant switch deviceffor' use with radar sighal receiving setshasbeen'disclosed-in t'he foregoing description,-variou's modificationswithin the spirit of the" in-- vention may occurt'o those skilled in thea-rtl'l Therefore;

itis intended that' n'o-limit'ations'beplaced-entireimie'ntic-nexcept'as defined by the scope of the appended clairris. Whatis claimed is: V I 1 1-. In cornbination' with a ra'dar signal receivingcircuit of a resistance capacity type and-having-a relatively long-5time constant, an-electronicfast-time constant switch con f nectedacrossthe output terminals of said receiving circuitto separate radarsignals from ground returns, said'switchf comprising a firstmultivibrator adaptedto yield a square wave output signal responsiveto atriggeringpulse supplied thereto, a' second multivibrator, a'diflerentiatin'g I circuit connected between the first'and secondmultivibra-* tors and'a'rranged to trigger the se'cond multivibrato'rresponsiveito said square' wave outputsignal ata predeter-':

mined time after the triggering pulse is applied'to the firstmultivibrator, saidsecond='multivibratorbeing arrangedto provide anoutput pulse of opposite polarity to that of the radar signal applied tothe receiving circuit; a'first rectifier and -a"low' value'resistorconnected'in series'across the output 2 terminals of said receivinglcircuit; a condenser and a-second receifier' connected in'series'across'the out put terminals" of the second multivibrator, and'asecondresistor connected between the respectivecommon termi naljunctions of the first rectifier and low-value resistorand the'condenserand secon'drectifier; saidrectifiers hav- 1 ingiterminals'of commonpolarity'connected to the re-" spective opposite terminals of'the secondresistor.

2. In combination with a'radarjreceivihgcircuit of the? resiStancecapacity typehaving a relatively 'longtimecon stant, means for rapidlydischarging the circuit in response to a ground returnsignal comprisinga first multivibrator;

a' secondmultivibrator, a differentiating circuitconnected. 7 betweensaid multivib'rators'mndearrangeditoltrigger the'{ V secondmultivibrator responsive to a ground return signal applied to the firstmultivibrator, said second multivibrator being arranged to yield aderived signal opposite in polarity to said ground return signal at apredetermined time after the application of the ground return signal, a

condenser and a first rectifier connected between the output of thesecond multivibrator and the output of the receiving circuit, saidrectifier being polarized to normally prevent discharge of the signalreceiving circuit, and a second rectifier connected to said condenserand arranged to discharge said condenser responsive to said derivedsignal and to cause discharge of said receiving circuit through saidfirst rectifier.

3. In combination with a signal receiving circuit of the type having arelatively long discharge time constant, a relatively low resistancedischarge circuit, a normally nonconducting rectifier connected betweensaid discharge circuit and signal receiving circuit, a relatively longtime constant charging circuit connected to said discharge circuit, asecond rectifier connected in circuit with said charging circuit andsaid discharge circuit, and means for discharging said charging circuitthrough said second rectifier in response to and at a predetermined timeinterval after the reception of a triggering pulse by said signalreceiving circuit, whereby the potential across said discharge circuitis reduced sufficiently to cause the firstnamed rectifier to conduct andto reduce the discharge time constant of the signal receiving circuit.

4. In combination with a signal receiving circuit of the type having arelatively long discharge time constant, a relatively low resistancedischarge circuit, a normally nonconducting rectifier connected betweensaid discharge circuit and signal receiving circuit, a relatively longtime constant charging circuit connected to said discharge circuit, asecond rectifier connected in circuit with said charging circuit andsaid discharge circuit, and a multivibrator circuit connected to saidcharging circuit and arranged to discharge said charging circuit throughsaid second rectifier in response to and at a predetermined timeinterval after the reception of a triggering pulse by said multivibratorcircuit, whereby the potential across said discharge circuit is reducedsufiiciently to cause the firstnamed rectifier to conduct and to reducethe discharge time constant of the signal receiving circuit.

5. In combination with a signal receiving circuit of the type having arelatively long discharge time constant, a relatively low resistancedischarge circuit, a normally nonconducting rectifier connected betweensaid discharge circuit and signal receiving circuit, a relatively longtime constant charging circuit connected to said discharge circuit, asecond rectifier connected in circuit with said charging circuit andsaid discharge circuit, and a multivibrator circuit connected to saidcharging circuit and arranged to apply a discharging potential to saidcharging circuit to cause said charging circuit to rectifier in responseto and at a predetermined time interval after the reception of atriggering pulse by said multivibrator circuit, whereby the potentialacross said discharge circuit is reduced sufiiciently to cause thefirstdischarge through said second named rectifier to conduct and toreduce the discharge time constant of the signal receiving circuit.

6. In combination with a signal receiving circuit of the long timeconstant type, means for reducing the time constant of the receivingcircuit at a predetermined time interval after the reception of atriggering pulse by the receiving circuit comprising a low resistancedischarge circuit, a rectifier connected between said discharge circuitand said signal receiving circuit, said rectifier being polarized toprevent current flow from said signal receiving circuit to saiddischarge circuit as long as a substantial potential exists across saiddischarge circuit, a source of potential connected across said dischargecircuit, a. second rectifier connected across said discharge circuit andpolarized to prevent current flow therethrough when said source has afirst polarity, means connected to said source of potential and arrangedto reverse its polarity in response to and at a predetermined timeinterval after the reception of said triggering pulse, and means wherebythe second rectifier then conducts and sufficiently reduces thepotential across said discharge circuit to cause the first rectifier toconduct.

7. In combination with a signal receiving circuit of the long timeconstant type, means for reducing the time constant of the receivingcircuit at a predetermined time interval after the reception of atriggering pulse by said signal receiving circuit comprising a lowresistance discharge circuit, a rectifier connected between saiddischarge circuit and said signal receiving circuit, said rectifierbeing polarized to prevent current flow from said signal receivingcircuit to said discharge circuit as long as a substantial potentialexists across said discharge circuit, a charging circuit including asubstantial capacitance in series with a substantial resistance and avoltage source connected across said discharge circuit, a secondrectifier connected across said charging circuit and polarized toprevent current flow therethrough when the potential across saidcapacitance has a first polarity, means connected to said voltage sourceand arranged to reverse the polarity of the potential across saidcapacitance in response to and at a predetermined time interval afterthe reception of said triggering pulse, and means whereby the secondrectifier then conducts and suificiently reduces the potential acrosssaid discharge circuit to cause the first rectifier to conduct.

References Cited in the file of this patent UNITED STATES PATENTS2,402,916 Schroeder June 25, 1946 2,462,859 Grieg Mar. 1, 1949 2,603,708Anger July 15, 1952 FOREIGN PATENTS 1,025,157 France Ian. 21, 1953 OTHERREFERENCES Radar System Engineering, vol. 1 of Radiation LaboratorySeries, published 1947.

